If c = number of Classical CDs, she has 3c Rock CDs, three times as many. Adding together 11 Pop, c Classical and 3c Rock CDs we have 35 CDs, so 11+c+3c=35. Therefore 11+4c=35. Subtracting 11 from each side: 4c=24, so c=6. That's 6 Classical and 18 Rock CDs.